At microwave frequencies of 1 GHz or more, amplifiers used in radio communication devices, mobile communication base stations, radar devices and the like require high power added efficiency.
The power added efficiency can be increased when the harmonic impedance seen from the output electrode of the semiconductor amplifying element toward the load is made close to the open-circuit impedance.
At microwave frequencies, the output impedance of a semiconductor amplifying element such as HEMT (high electron mobility transistor) and FET (field effect transistor) is capacitive at the fundamental. Efficiently extracting the signal amplified by the semiconductor amplifying element needs impedance matching between the output impedance of the semiconductor amplifying element and the external load at the fundamental.
Thus, for impedance matching between the semiconductor amplifying element and the external load, the impedance seen from the semiconductor amplifying element toward the load at the fundamental needs to be a desired inductive impedance. On the other hand, to increase the power added efficiency, the impedance seen from the semiconductor element toward the load at the second harmonic needs to be near the open-circuit impedance.
The impedance seen from the semiconductor amplifying element toward the load at the second harmonic can be set near the open-circuit impedance by providing a higher harmonic processing circuit using a stub near the chip. However, in this case, a matching circuit is cascaded via the higher harmonic processing circuit. This increases insertion loss and narrows the bandwidth.